1. Field of the Invention
The present invention relates to method and apparatus for measuring concentration of a solution detecting a spectroscopic absorbance by the solution.
2. Description of the Related Art
In general, spectroscopic methods are widely used to determine the concentration of a chemical substance, and their fundamental principle is based on Lambert-Beer's law, which is expressed by the following formula (1). EQU A=abc=log (l/T) (1)
where
A: the absorbance of the sample at a characteristic absorption wavelength .lambda. of a specific component, PA1 T: the transmittance of the sample at the characteristic absorption wavelength .lambda. of the specific component, PA1 a: the absorptivity of the specific component at the characteristic absorption wavelength .lambda. of the specific component, PA1 b: cell length or pathlength, PA1 c: the concentration of the component in the sample. T is expressed by the following formula (2). EQU T=I.sub.t /I.sub.0 ( 2) PA1 I.sub.0 : the intensity of the incident light having the characteristic absorption wavelength .lambda., PA1 I.sub.t : the intensity of the transmitted light having the characteristic wavelength .lambda.. PA1 (1) The problem on the optical system. PA1 (2) Problem on calibration curve PA1 (3) Problem on spectrum analysis
where
If the sample is liquid, a cell is necessarily used. If the same cell is used for a sample having unknown concentration as for a standard sample having known concentration, the values of the absorptivity a and the cell length b need not be obtained. In fact, a and b are regarded as constant, and if k=ab, then formula (1) is simply expressed as the following formula (3). EQU A=kc (3)
Once the value of the above constant k is determined by measuring the absorbance of a standard sample, the unknown concentration of the component in a sample is calculated and estimated by measuring the absorbance of the sample.
However, in order to implement the above method based on Lambert-Beer's law and to obtain data with precision, accuracy, reproducibility, and reliability, the following various problems have to be solved.
A photometric signal varies with drifts caused by changes in the intensity of the light source and the fluctuation of the sensitivity of the photo detector. Unless these drifts are reduced and canceled, obtained data can not be trusted. Therefore, the light source and the photo detector have to be stabilized to reduce and prevent the drifts.
However, since the radiation from of a light source varies nonlinearly and deteriorates with time, its calibration is not easy. Even if some measures are taken under less than ideal condition, it is not guaranteed that there is no need of blank correction for a long time.
In particular, in process measurement of on-line and in-line systems, frequent blank correction is not only cumbersome and inconvenient, but also difficult.
Therefore, the stabilization of the light source and photo detector costs much but can not guarantee good results.
On the other hand, if a sample is liquid, it is necessary to perform measurement using a cell. In this case, apart from the true absorption of light by the sample, there are reflected components on the surfaces of cell windows due to the differences of the refractive indices among the cell window material, the sample, and the atmosphere, absorbed components by the cell windows themselves, a scattering component due to flaws of the cell, an absorbed component by stains, and a scattering component due to dust floating in the sample or suspensions in the sample. How to remove and cancel these components is an important factor in determining the precision and accuracy of the measurement.
Between the concentration of a specific component in the sample and the sample's absorbance A there is a relationship expressed by the formula (3), which is a determining equation for concentration. However, actual measurement is not so simple, since device-dependent constants and experimental and measurement errors are involved. Therefore, a so-called calibration curve is drawn in the following way.
Many standard samples having known different concentrations are prepared considering the range of working concentrations. Then the constant k is determined by means of the least-square method to obtain the calibration curve. If the equation of the relationship between the absorbance and the concentration does not passes the origin, then an offset k.sub.0 is added to the formula (3) to use the formula A=kc+k.sub.0.
The calibration curve is sometimes not a straight line as in the above formulas, and a curve is used. Also, in case of a sample of a multi-component system, multivariate analysis is used.
The calibration curve is device-dependent, and compatibility and universality are not guaranteed. Also, an calibration curve once determined can not be used for a long time. The reliability of measurement is not maintained unless the calibration curve is regularly checked and corrected.
When a lamp of the light source or a cell is replaced, the judgment whether an established calibration curve is correct or not is not simple, but the recalibration is often required.
In the spectrum of a sample of a liquid system, particularly an aqueous solution, a background signal of an aqueous solution is superimposed. The baseline shifts among the samples with drifts due to optical and electrical systems and often oblique and bent.
In order to correct the baseline shift, the so called two-wavelength method and the baseline method are used. In the two-wavelength method, a characteristic absorption peak wavelength is taken as a measuring wavelength, and a wavelength at the slope of a side of the peak or an isobestic wavelength is taken as a reference wavelength to obtain the ratio of intensities. In the baseline method, two wavelengths at the slope of both sides of the peak are taken as reference wavelengths, and a baseline is drawn at a slope of the reference wavelengths to measure the height of the peak from the baseline. In short, the difference between the two methods is how to set a reference level for the height of the peak. When the background signal of a solvent is superimposed, the baseline method is superior, but it costs more, since it uses three wavelengths.
Both the two-wavelength method and the baseline method are excellent techniques. However, these methods assume that the ratio of intensities of light source at a reference wavelength and at a measuring wavelength does not change. The profile of the spectrum of a light source is stable in a short time. But, in a long time, this ratio of intensities varies, and blank correction is required. If the light source is replaced or deteriorates, blank correction should be made again.